High speed transmission systems (in which the data rates exceed 10 Gb/s) for cost-sensitive applications, such as a metropolitan area network have attracted extensive interest due to the explosive data traffic growth in such applications.
Conventional optical data communication systems include two types of transmission combinations. The first type is a low-cost direct modulation type, according to which data is transmitted by directly modulating the current of a laser transmitter. The second type include a laser biased at constant current and an external modulator that converts the continuous wave light into a data coded pulse train with the right modulation format. Moreover, in recent years it has been suggested to use coherent reception communication systems which could combat using DSP linear channel impairments instead of a single photodiode and allow the use of advanced modulation formats.
The key limiting factor of using direct modulation for high speed communication systems is the transmitting laser frequency chirp (a measure of the change in laser transmitter's optical frequency as the transmitter is modulated). In this case the transmitted optical signal is amplitude modulated. Similarly, the receiver uses a low-cost photodiode for direct detection of the amplitude, so as to extract the transmitted data. In this case, the levels of linear and non-linear phase distortions is unacceptable.
Linear distortions are generally related to the linear distortion introduced by the optical channel, such as Chromatic Dispersion (CD—the dependency of the phase velocity of an optical signal on its wavelength). Other distortions, such as chirp and phase-noise are of a non-linear nature and/or statistical. Therefore, even though systems using direct modulation and direct detection are low-cost system, the linear, non-linear and statistical distortions are convoluted and since direct detection is limited only to amplitude detection, all the data embedded within the phase of the optical signal is lost and the resulting distortion is very large and as a result, the transmitted data cannot be detected.
Regarding the use of external modulator with coherent detection the transmitting laser frequency chirp is very low due to the fact that modulation is done by an external modulator, such as a Mach-Zehnder Modulator. Coherent reception allows efficiently compensating most types of distortions, including phase distortions and therefore, systems of this type are suitable for much longer range optical channels. However, such systems are very expensive.
All the methods described above failed to propose a medium cost system employing a digital signal processing (DSP) process that can compensate for both laser frequency response and laser frequency chirp in an optical communication system, based on laser rate equations and fiber parameters.
It is therefore an object of the present invention to provide a method that allows using a low cost transmitter implemented by standard off-the-shelf lasers (such as Distributed Feedback (DFB) lasers), without requiring any external modulators or optical filters with coherent reception using advanced modulation formats, such as optical OFDM, using direct modulation.
It is an object of the present invention to provide a low cost transmitter which allows using direct modulation for very long ranges of optical channels.
It is still a further object of the present invention to use a low cost transmitter while still allowing minimizing the effect of phase distortions.
Other objects and advantages of the invention will become apparent as the description proceeds.